linux-kernel - Re: [PATCH 09/11] x86/pti: enable global pages for shared areas

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Message-ID: <503339cf-7e54-0888-1767-c8ac87ce2130@linux.intel.com>
Date:   Mon, 2 Apr 2018 13:41:57 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jürgen Groß <jgross@...e.com>,
        the arch/x86 maintainers <x86@...nel.org>, namit@...are.com
Subject: Re: [PATCH 09/11] x86/pti: enable global pages for shared areas

On 04/02/2018 10:56 AM, Linus Torvalds wrote:
> On Mon, Apr 2, 2018 at 10:27 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
>> +       /*
>> +        * The cpu_entry_area is shared between the user and kernel
>> +        * page tables.  All of its ptes can safely be global.
>> +        */
>> +       if (boot_cpu_has(X86_FEATURE_PGE))
>> +               pte = pte_set_flags(pte, _PAGE_GLOBAL);
> So this is where the quesion of "why is this conditional" is valid.
> 
> We could set _PAGE_GLOBAL unconditionally, not bothering with testing
> X86_FEATURE_PGE.

I think we should just keep the check for now.  Before this patch set,
on !X86_FEATURE_PGE systems, we cleared _PAGE_GLOBAL in virtually all
places due to masking via __supported_pte_mask.

It is rather harmless either way, but being _consistent_ (by keeping the
check) with all of our PTEs is nice.